In a cellular system, a method for detecting the position of a mobile terminal has been proposed (for example, refer to Japanese Patent Document Cited 1). This method comprises calculating time differences of arrival of signals transmitted from base stations to the mobile terminal (that is, time differences of propagation of the signals from the base stations to the mobile terminal T1-T2 and T3-T2), multiplying the propagating time differences by velocity of light, thus calculating differences between the propagation distances of the signals from the base stations to the mobile terminal:D1−D2=c(T1-T2); andD3−D2=c(T3-T2).
In a wireless LAN system as well, a method for detecting the position of a terminal has been proposed (for example, refer to Non-Patent Document Cited 1). This method comprises calculating time differences of arrival of a signal transmitted from the terminal and received at base stations (that is, differences between times when the base stations received the signal, Ti−T1), multiplying the time differences of arrival by velocity of light, thus calculating differences between the signal propagation distances from the terminal to the base stations:{|P−Pi|−|P−P1|}=c(Ti−T1), i=2, . . . , n.
FIG. 9 is a diagram showing the topology of base stations in such a conventional wireless communication system.
Base stations 1 to 3 can perform uplink communication (a terminal transmits a signal and a base station receives the signal) and downlink communication (a base station transmits a signal and a terminal receives the signal) with terminals present in their areas 11 to 13. In general, in a wireless communication system, base stations are deployed so that as small numbers of base stations as possible can serve for communication with a terminal 20 in a location. That is, the base stations are deployed so that the overlap portions of the areas 11 to 13 are minimized.
FIG. 10 is a block diagram showing the configuration of a base station for use in the wireless communication system shown in FIG. 9. The base station is representative of base stations in an instance where a terminal position is detected, based on the time differences of arrival of a signal transmitted from the terminal and received at the base stations, as described in Non-Patent Document Cited 1.
A signal received by an antenna 100 is input through a circulator 101 to a radio signal receiver 102. The radio signal receiver 102 converts the signal to a baseband signal and a baseband signal processing unit 103 performs demodulation, decoding, and error correction of the baseband signal. Then, a signal processing unit for position detection 106 performs signal processing required for position detection, such as received signal capturing, a delay profile calculation, and path detection.
FIG. 11 is a flowchart of a communication data rate setup process that is applied in the wireless communication system shown in FIG. 9.
In general, in the wireless communication system, uplink and downlink communication data rates are set when communication is initiated and during the communication. By a communication data rate required for quality of signal, traffic, and contents of communication (for example, voice, still picture, moving picture, data, etc.) on a radio channel between a terminal and a base station, the communication data rate of the radio channel is determined (step 310). Specifically, for example, if the quality of signal is bad, traffic is large, or the required communication data rate is low, then the communication data rate is set low. Conversely, if the quality of signal is good, traffic is small, or the required communication data rate is high, then the communication data rate is set high.
This communication data rate setup process is performed at the base station and the terminal. For uplink communication and downlink communication, different communication data rates may be set up or a same communication data rate may be set up.
[Japanese Patent Document Cited 1] JP-A No. 181242/1995
[Non-Patent Document Cited 1] Ogino Atsushi, et al. (five co-writers) “Integrated Wireless LAN Access System (1) Exploring a Position Detection System” in collected lecture papers B-5-203, p. 662, for the 2003 general conference of the Institute of Electronics, Information and Communication Engineers
The above-discussed conventional wireless communication system is designed such that the uplink cover area of a base station (area where the base station can receive a signal from a terminal) and the downlink cover area (area where a signal transmitted from the base station arrives at a terminal) of the base station are identical. For example, if the terminal 20 is present in the location as shown in FIG. 9, the terminal 20 is present within the uplink cover area of the base station 1, but not present within the uplink cover areas of the base stations 2, 3. Therefore, the base station 2 and base station 3 cannot receive an uplink signal transmitted by the terminal 20 (or the base stations 2 and 3 can receive, but receive only a poor quality signal from the terminal). Consequently, only the base station 1 can receive the uplink signal transmitted by the terminal 20.
To implement the above-mentioned method for detecting the position of a terminal, based on the time differences of arrival of a signal transmitted from the terminal and received at base stations (refer to Non-Patent Document Cited 1), at least three base stations have to receive an uplink signal transmitted by the terminal. To detect the position of the terminal 20 by the position detection method using such an uplink signal, thus, additional base stations 4 and 5 having uplink cover areas where the terminal 20 is present need to be installed, for example, as is shown in FIG. 12.
Requirement for communication purpose only is simply that any possible position of the terminal falls within the uplink/downlink cover areas of one base station. However, requirement for position detection of a terminal is that any possible position of the terminal must fall within the uplink cover areas of three base stations. Therefore, the number of base stations throughout the system required to realize terminal position detection increases three times as many as the number of base stations required for only communication between a base station and a terminal.
Accordingly, for a wireless communication system which provides for terminal position detection, cost needed for its equipment rises, which in turn poses a problem of rise in the cost of a location information service (service fee) that informs a terminal user of its detected position.